A digital imaging device generally comprises an image sensor and processing means. Contemporary image sensors can be produced with the aid of various technologies often based on monocrystalline-silicon substrates, namely sensors using CCD (“Charge-Coupled Device”) technology, MOS (“Metal Oxide Semiconductor”) and CMOS (“Complementary MOS”) sensors, but also for certain more specific applications regarding technologies based for example on thin layers of amorphous silicon (a-Si:H), or indeed other materials or substrates. Independently of the technology used, in each category, the image sensor comprises photosensitive dots, also called pixels, organized in rows and columns so as to form a matrix. Each pixel is able to convert the electromagnetic radiation to which it is exposed into electric charges and comprises a charge collector element collecting electric charges under the effect of an incident photon radiation. The electric charges can notably be generated by photosensitive elements associated with the charge collector elements. Traditionally the pixels comprise one or more photosensitive elements making it possible to detect electromagnetic radiation with a wavelength in the visible range or close to the visible range. In the medical field and in the industrial field, where X-ray or γ-ray radiations can be used, it is usual to interpose a radiation converter between the source of the radiation and the image sensor. Such a converter may for example be a scintillator or a photoconductor, respectively converting the incident electromagnetic radiation into a radiation of greater wavelength, typically that of visible light, or into electric charge. Stated otherwise, the scintillator emits photons under the effect of incident radiation while the photoconductor generates charge carriers under the effect of incident radiation. For these reasons the conversion by scintillator is commonly called indirect conversion and the conversion by photoconductor material, direct conversion (by reference to the electrical output signal).
In a CCD technology image sensor, the electric charges are read by being moved from pixel to pixel up to a charge reading circuit placed at an end of the matrix. In an image sensor produced on the basis of MOS or CMOS technological pathways, the means for reading the electric charges are in general partially integrated into the pixels. The conversion of the electric charges into electrical signals actually takes place inside the pixels. These electrical signals are read row by row at each end of the columns of pixels. For this purpose, each pixel comprises at least one element having a control or processing function (e.g. circuit breaker, reset, amplification) in addition to the photosensitive element or elements or to the charge collector element. In devices other than CCDs, the pixels are commonly classed into 2 large categories namely, on the one hand, passive pixels in which the charges are transferred outside the pixels without additional processing and, on the other hand, active pixels which integrate processing functions that are slightly more sophisticated locally at the level of the pixels (e.g. amplification). The image sensor also comprises row conductors linking the pixels row by row, and column conductors linking the pixels column by column. The row conductors are connected to an addressing circuit, also called a row addressing block, and the column conductors are connected to a reading circuit, also called a column reading block. The row addressing blocks and the column reading blocks are arranged at the periphery of the matrix, on two perpendicular sides. The row addressing block makes it possible to actuate the circuit breaker elements of the pixels row by row, and the column reading block makes it possible to read the electrical signals on the column conductors. The processing means of the imaging device make it possible to process the raw signals recovered on the column reading block.
In the field of X-ray imaging, image sensors employing MOS or CMOS technology are known but little used at this juncture, with the exception of the intra-oral dental field, notably because of the limited dimensions required for this application. This results from the marriage of two factors. The first factor is that the X-ray radiations cannot be focused over distances compatible with the applications, that is to say typically of the order of a meter. Consequently, the dimensions of the image sensor must be at least equal to those of the object to be imaged. The second factor is that MOS and CMOS image sensors are produced on silicon wafers whose dimensions are relatively restricted. These wafers mostly have a diameter of between 100 millimeters (mm) and 300 mm. A MOS or CMOS image sensor of rectangular shape produced on a silicon wafer therefore exhibits dimensions of markedly less than 300 mm. Thus, numerous organs of the human body cannot be imaged by such a sensor. Image sensors using silicon wafers of greater diameter would have a prohibitive cost. One solution consists in abutting several CMOS image sensors alongside one another in one or two directions, as is done with detection matrices employing amorphous silicon technology (a-Si:H). However, for the image sensors, the abutting of the pixels of a first sensor with those of a second sensor is heavily penalized, or indeed prevented on the sides where the row and/or column addressing blocks are situated, because of the difficulty of sensing a representative signal in this zone. Furthermore, the driving of the various sensors with one another is rendered complex. Another drawback of the production of rectangular-geometry MOS or CMOS image sensors on silicon wafers is that the zones of defects are generally denser when approaching the exterior edge of the wafers. It is therefore preferable to make an exclusion zone at the periphery of the silicon wafers, thereby further restricting the surface area utilized.